The assessment of arterial blood and tissue oxygen saturation has been shown to be critical for monitoring, diagnosing, and treating acute cardiovascular deficiencies, for example but not limited to low blood oxygen saturation due to chronic obstructive pulmonary disease (COPD) or, in extreme cases, exsanguinations. Further, the time line over which physiological changes occur is indicative of underlying cardiovascular deterioration. While techniques exist that allow qualitative monitoring of the level of arterial blood and tissue oxygen saturation, the current medical practice appears to lack a tool for quantitative monitoring of deteriorating cardiovascular status from the initial cardiovascular event or acute trauma through arrival at a medical facility.
Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (“λ”). Historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism.
Later the concept of spectroscopy was expanded greatly to comprise any measurement of a quantity as a function of either wavelength or frequency. Thus, it also can refer to a response to an alternating field or varying frequency (“ν”). A further extension of the scope of the definition added energy (“E”) as a variable, once the very close relationship “E”=“hν” for photons was realized (“h” is Planck's constant). A plot or measurement of the response of a material or structure as a function of wavelength—or more commonly frequency—is referred to as a spectrum.
ISO Standard number 80601-2-61:2011 states, on page 34: “Current technology requires an adequate concentration of haemoglobin, a pulsatile change in blood flow, and light transmission through a tissue bed to approximate the in vivo haemoglobin oxygen saturation. PULSE OXIMETER EQUIPMENT is not typically capable of functioning effectively during cardiopulmonary bypass or at extreme low-flow states, and is not at present intended as a means for the measurement of blood flow or blood volume. Given these limitations, PULSE OXIMETER EQUIPMENT does not provide precise measurements of arterial haemoglobin saturation. The presently marketed in vivo PULSE OXIMETER EQUIPMENT is not a replacement for measurement of blood samples by in vitro optical oximeters. The values derived from pulse oximetry are not a measurement of blood or solid-tissue oxygen tension. Pulse oximetry provides no direct indication of oxygen delivery to tissue, or of tissue oxygen consumption.”
Recent theoretical developments have pointed to the potential advantage of a spectroscopic device that uses multiple wavelengths of light to perform oximetry. Spectroscopic devices (including oximeters) using multiple well-chosen wavelengths have improved accuracy without requiring the use of extremely large devices. There remains a need for a method to accurately select the optimal multiple wavelengths to enable a device that can acquire and process data for use in quantitative cardiovascular measurements. The multiple selected wavelengths are also beneficial in devices used for quantifying other material components from the acquired results, and optimization of the process of accurately and completely distinguishing between or among the analytes (referred to herein as analyte contrast).
Furthermore, there remains a need for improved measurement devices configured to operate to measure relative and/or absolute concentrations of molecular analytes in a sample. Conventional models have ignored path length and especially wavelength-based variations in a path length of an electromagnetic wave traversing a sample being measured. Incorporating this information can increase the sensitivity and accuracy of the measurements, providing a wealth of derived property evaluations. Some of the material components that are quantified include molecular analytes. Non-limiting examples of these molecular analytes are cells, proteins, hemoglobin, glucose, lipids, chromophores, water, pH, and gases such as hyperpolarized gases, carbon dioxide, carbon monoxide, and oxygen. Due to short transit times of signals, and the rich information conveyed in the variations in the impulse and therefore the step response of a sample, the accurate and fine-grained measurement of the step response by a spectrometric system is difficult but valuable.
Given the capability to utilize the fine-grained information for more accurate measurements and assessments, there is a need for systems, methods, and apparatus that incorporate the ability to accurately measure very small signal levels and very short transit time thereby providing the details of the collective differential path lengths of the transmitted signals.